Coating equipment



Nov. 9, 1943. T. A. KAUPPI ETAL 2,334,102

COATING EQUIPMENT Filed Aug. 26, 1940 Ffam Supply 23 To Wilda- INVENTORS70/70 A. KAl/PP/ 90652 W. ZOIDEIMAA/ BY BIC/I120 J, MtCU/IG M Kama!ATTORNEYS Patented Nov. 9, 1943 uurrso coa'mzo murmur oivoA.Kanppi,Roger W. Kolderman, and Bichi ard S. McGliu'I, Midland, Mich., assignorsto The Dow Chemical Company, Midland, Mich., a corporation of MichiganAppiication August 26, 1940, Serial No. 34,242.

-2 Claims.

Thisinvention relates to a specific improvement in coating equipment ofthe rotating doctor a rod type of .particular use for coating flexiblesheet material. It relates especially to rotating doctor rods for use inapplying hot, molten coating compositions to flexible sheet material.

As an instrument for removing excess coating composition and fordistributing a coating more or less uniformly over flexiblesheetmaterial, the

. doctor knife is well known. In the case of lac quers and similarliquid coating compositions which are capable of smoothing themselves byflowing after having been doctored, the knife is a satisfactoryspreading instrument. When, however, the coating composition is of waxyor resinous character, and is applied from the molten state, the doctorblade almost invariably leaves longitudinal scratches on the surface ofcoated sheet material which has passed across the blade. Thus, it hasbeen stated that the doctor knife is satisfactory for spreading basecoats but not for the surface or finish coats when applying pluralcoatings. It often happens that only one coat is. to be applied, andthat the doctor knife is not adapted to satisfactory production of thedesired smoothness in the surface coating.

In addition to the doctor knife, and as a substitute therefor, doctorrollers have been proposed and employed. It has been proposed to usesuch rollers both driven and undriven. The driven rollers of the pasthave been said tobe objectionable in that they fail to deposit evencoatings, and articles coated thereby have a splotchy surface. It hasbeen found that in most instances this is true, and that idle orundriven rollers, whether employed singly or in pair to produce a steamroller action on the fresh coating, are also ineffective to produce asmooth blemish-free surface coating when operating on hot-melt coatingsof the resinous type or of the wax-cellulose derivative type to bedescribed hereinafter. The idle rollers heretofore specificallydescribed have been from 1 to 2 inches in diameter (see '0'. S. Patent2,196,894, Boice) while both idle and driven rollers, such as areemployed in the Mayer paper waxing machines, have generally been atleast inch, and usually over /2 inch in diameter. The tendency has beentoward the use of larger rollers to assure less frequent recurrence ofthe reproduction of roller 7 surface defects on the coated sheet.

A particular problem arises when coating thermoplastic sheet material.When thermoplastic foils are to be given a thin coating, especially withmolten compositions, the tension ordinarily applied to paper in a likecoating operation stretches and distorts the foil. Even when using meltswhich can be applied at or below 100 0.,

'such thermoplastic foils must be coated while under very low tension.Under such circumstances neither the customary doctor knives nor thecustomary doctor rollers are satisfactory, as they all give uneven,streaked coatings. Both the doctor knife and the doctor roller, asordinarily employed, pick up and retain in the pool of coating materialon which they operate any minute particles of grit or other substancewhich is hard enough to abrade the freshly deposited coating,

and these retained foreign bodies augment the I already undesirablescratching effect of the spreading instruments themselves.

It is, accordingly, among the objects of the present invention toprovide a coating equip.- ment whereby smooth, unblemished thin coatingsmay be applied to'fiexible sheet material. It is another object toprovide a means as aforesaid, operable to coat uniformly a thermoplasticfoil without subjecting the latter to such tension as will distort thesame at the coating temperature. A further object is to provide acoatinglevelling device particularly adapted to operate on coatings ofthe resinous or waxy melt type to smooth and distribute the same withoutproducing any material surface blemishes thereon. Specific objectsinclude the provision of a particular form of doctoring device capableof accomplishmg the foregoing ends, and the provision of a preferredmode of employing such a device in a coating operation. These and otherobjects, and a mode whereby they may be realized, are embodied in thefollowing description of the invention.

To the accomplishment of the foregoing and related ends, the invention,then, consists of the means hereinafter fully described and particularlypointed out in the claims, the annexed drawing and the followingdescription setting forth in detail'certain means for carrying out theinvention, such disclosed means illustrating, however, but some of thevarious ways in which the principle of the invention may be used.

In the said annexed drawing, wherein like numerals are employed todesignate like parts:

Figs. 1 and 2 are diagrammatic representations, in side sectionalelevation, of typical coating apparatus for applying surface coatings toflexible sheet material, embodying the improvement of the invention;

Figs. 3 and 4 areperspective views of two different modifications ofeacoating machine accessory, embodying the doctor rod of the invention;

Fig.5 is a cross-sectional elevation taken along the line 5-5 of Fig. 3,illustrating one particular assembly embodying the doctor rod: and

Fig. 6 is a cross-sectional elevation taken along the line 66 of Fig. 4illustrating one assembly alternative to that shown in Figs. 3 and 5.

According to the invention, one or a plurality of mechanically driven,rotating and heated cylindrical doctor rods rigidly supported throughouttheir length, are employed to smooth and distribute coatings on flexiblebases, each such rod being of a diameter not to exceed fig inch, andpreferably being 54; inch or less in diameter. The rods may be driveneither with or counter to the direction of travel of the sheet beingcoated, and for best results, when, as in the preferred embodiment, twosuch rods are employed on each surface being coated, the first is drivencounter to the sheet and the second in the same direction as that of thesheet. Numerous tests have shown that rods of $4 inch diameter andlarger have many or all of the failings inherent in the rollersdescribed in the prior art, while at or below the critical diameter ofinch, they operate to form a smooth, flawless surface finish infulfilment of the objects hereinb'efore stated. The suprisingly criticalnature of this dimensional limitation on the rotating rod diameter makesnecessary the provision of means for rigidly supporting the rodthroughout its length, as without such support it has been found thatthe shaft tends to flex when rotated, departing from the coating plane,and failing to exert any continuous smoothing or leveling of the appliedcoating. Provision should be made for heating the rotating rod,especially when operating on waxy or resinous melt coatings, so that anyof the composition which may already have congealed before reaching thesmoothing rod may be softened and distributed in the instant of passageacross the rod.

A preferred mode of carrying out coating operations utilizing therotating doctor rods. of the present invention will be described withreference to the accompanying drawing, and with particular respect tothe applicationof waxy or resinous melt coatings on flexible sheetmaterial such as paper, Cellophane or ethyl cellulose foil. Two coatingdevices are illustrated diagrammatically in the drawing. That in Fig. 1employs a roller coating operation and that in Fig. 2 coats by immersionof the sheet in a coating bath. A flexible sheet material It may beunwound over rollers II and I2, and past applicator roll l3 which iscontinuously acquiring a thin coating of the fluid composition l5 fromroller II which dips into the bath contained in a suitably heated pan i6having a drip catcher l1 extending under rods 19 and 20. The sheet lacquires a thin coating from roller l3, and then passes around tensionroller l8 and over a pair of rotating doctor rods l9 and 20, to be morefully described hereinafter. After passing the doctor rods, the coatedsheet 23 is passed around and over chill rolls 2! and 22 to congeal thesmooth coating, Alternatively, sheet ID are to be coated, the said sheetI is passed directly into and beneath the surface of the fluidcomposition i contained in the suitably heated pan l6 as shown in Fig.2. The flexible sheet is withdrawn from the bath after passing aroundroller 24 and then both sides of the sheet are exposed to the smoothingaction of a plurality of doctor rods l9, I9, 20 and 20', operating inpairs disposed on each side of the sheet. The said sheet with itssmoothed double coating is passed through a cooling zone and thence overa cold roller 22 to a suitable wind up.

The rotating-doctor rods illustrated diagrammatically in Figs. 1 and 2are those shown in perspective in Fig. 4 and in cross-section in Fig. 6.In said Figs. 4 and 6, the rotatable doctor rod or when both sides ofthe flexible l9, which is inch or less in diameter, is rigidly supportedthroughout its length by an extra heavy steam tube 34 having a suitablyprofiled channel 38 machined in the wall 33 thereof. The said wall 33 ispreferably further machined to provide scraper edges 35 to remove anyexcess coating which may tend to adhere to rod I9, and to preventtransport of the coating into channel 38. In assembly, rod l9 having abearing 30 at each end and a driving means, such as a belt driven pulley39, whereby it may be rotated, is

supported in the channel 38 of steam tube 34. Steam may be admitted tothe tube through valve 40 and pipe 31. Channel 38 is machined to providea close fit between rod i9 and tube 3!, much like the fit between ashaft and a journal hearing. The spacing in Fig. 6, like that in Fig. 5,is greatly exaggerated for ease of understanding. The rod l9 andsupporting tube 34 are mounted in suitable end supports 3| provided withopenings for bearing 30 and for the steam coupling 31. In operation, rodI9 is rotated while steam or other heating fluid is passed through thetube 34.

Another assembly of the rotating rods of the present invention isillustrated in Figs. 3 and 5. The rod I 9 is supported longitudinally'byrigid member 21, to which are secured apair of rigid scraper blades 26having knife edges 32 in contact with the rod l9, furnishing lateralsupport and rigidity thereto. The blades 26 are mounted on bar 21suitably by means of bolts 28. In a preferred embodiment, bar 21 andblades 26 are electrical and thermal conductors, which may be heated,for example, by means of resistance bar 29 mounted on the rigid member21, and heated by current supplied through leads 36. This type ofassembly is mounted on suitable end supports 3| which are preferably ofinsulating material, the end of rod l9 extending through the bearing 30in support 3| and being driven in the desired direction by the pulley39. The support 21 and scraper blades 26 hold rod l9 rigidly in line,preventing any whi or flexing thereof during its rotation against theflat sheet being coated.

When operating a coating device embodying the small rotating rods of thepresent invention, especially when coating thermoplastic foils under lowtension to avoid distortion and wrinkling, the first rod or group ofrods oper-- ating on a coated face of the foil is preferably rotatedagainst the direction of travel of the sheet, at a speed sufljlcient todo all the leveling and scraping required to bring the depositedcoatingto the required thickness. The second rod or group of rods is preferablyrotated in the direction of travel of the sheet but with a surface speedless than the linear speed of the sheet, thus smoothing the coating.

A smooth, flawless coating was applied to one face of an ethyl cellulosefoil, using a set-up similar to that shown in Fig. 1, using a waxy meltas the coating composition. The melt consisted of Per cent Ethylcellulose 10 B'eckacite 2000 (pure phenolic resin) 13 Stabellite A-1(hydrogenated rosin) 5 Paraflin wa 28 Opal wax (hydrogenated castor oil)44 The ethyl cellulose film was moved through the machine at 60 feet perminute. The doctor rods each were of V inch in diameter, the firstrotating against the foil. at 250 revolutions per minute and the secondrotating with the rbn at 60 rollers, e. g; of inch to 2 inches diameter,were substituted for the rods here employed, blotchy and streakedcoatings were usually obtained reregardless of whether the rollers wereidle or driven.

For best results the rod employed should be sufllciently hard andabrasion-resistant to avoid being scratched byv grit particles which maycome in contact therewith during coating operations. Usually a rod ofBrinell hardness over 500 is preferred, though this is not an invariablyessential limitation.

We claim:

1. In apparatus for coating flexible sheet ma- 'R. P. M. The finishedarticle had'a smooth V flawless coating 0.0001 inch thick. When largerterial, the combination of a rotatable smooth doctor rod of diameter notexceeding inch disposed transversely of the path of the'sheet materialto be coated, means to support the doctor rod rigidly throughout itslength, means for heating' said rod and means forrotating the rod.

2. In apparatus for coating flexible sheet ma.- terial, the combinationof a plurality of rotatable smooth doctor rods of diameter not exceedinginch disposed transversely of the path of the sheet material to becoated,- means to support each of the rotatable doctor rods rigidlythroughout its length, means for heating said rods and means for drivingthem independently of one another.

TOIVO A. KAUPPI. ROGER W. KOLDERMAN. RICHARD S. McCLURG.

